Semiconductor device employing buried insulating layer and method of fabricating the same

ABSTRACT

A semiconductor device employs an asymmetrical buried insulating layer, and a method of fabricating the same. The semiconductor device includes a lower semiconductor substrate. An upper silicon pattern is located on the lower semiconductor substrate. The upper silicon pattern includes a channel region, and a source region and a drain region spaced apart from each other by the channel region. A gate electrode is electrically insulated from the upper silicon pattern and intersects over the channel region. A bit line and a cell capacitor are electrically connected to the source region and the drain region, respectively. A buried insulating layer is interposed between the drain region and the lower semiconductor substrate. The buried insulating layer has an extension portion partially interposed between the channel region and the lower semiconductor substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.2003-93437, filed Dec. 18, 2003, the disclosure of which is herebyincorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a semiconductor device and a method offabricating the same, and more particularly, to a DRAM cell employing anasymmetrical buried insulating layer and a method of fabricating thesame.

2. Discussion of the Related Art

A semiconductor device widely employs a discrete device such as a MOStransistor as a switching device. With the increase of high integrationof the semiconductor device, the MOS transistor is gradually scaleddown. As a result, a channel length of the MOS transistor is reduced anda short channel effect (SCE) becomes a problem. Therefore, a channel ionconcentration should inevitably be increased in order to reduce theshort channel effect However this causes an increase of a leakagecurrent, thereby deteriorating refresh characteristics.

Therefore, transistors having an SOI structure are widely studied toimprove the short channel effect. The SOI structure includes a lowersemiconductor substrate, an upper silicon pattern, and a buriedinsulating layer interposed between the lower semiconductor substrateand the upper silicon pattern, and insulating the lower semiconductorsubstrate from the upper silicon pattern. The transistor having the SOIstructure has capabilities to reduce a short channel effect and aparasitic capacitance, and operates at high speed while powerconsumption is small. However, it has a problem of a floating bodyeffect such as a kink effect.

In order to solve the problems related with the floating body effect, anew method has been introduced and studied on a technique ofelectrically connecting the upper silicon pattern region and the lowersemiconductor substrate. Further, a method of electrically connectingthe upper silicon pattern region and the lower semiconductor substrateis disclosed in U.S. Pat. No. 6,429,091, titled “Patterned BuriedInsulator” to Chen, et. al.

In the method disclosed in U.S. Pat. No. 6,429,091, a mask is formed ona semiconductor substrate, and buried doping regions are formed undersource/drain regions. Selectively etching the doping regions, andfilling an insulator, a buried insulating layer is formed. Then, atransistor has the source/drain regions formed on the top of the buriedinsulating layer. As a result, patterned buried insulating layers areformed interposed under the source/drain regions, thereby reducing ajunction leakage current and improving a floating body effect.

In order to further reduce the junction leakage current, the buriedinsulating layers need to be extended under the gate electrode, spacedapart from one another. In the above method, in order to form the buriedinsulating layers extended under the gate electrode, it is necessary toform a mask having a width smaller than that of the gate electrode.However, it is quite difficult to form patterns having a smaller widththan that of the gate electrode due to the demand of highly integratedsemiconductor devices. Thus, it is also difficult to make buriedinsulating layers aligned with a smaller gap than the width of the gateelectrodes.

Further, in the above method, the gate electrodes may be misaligned withthe buried insulating layers. The DRAM device typically has twotransistors and two cell capacitors inside one active region. That is,in the DRAM device, two adjacent cells on one active region may bereferred to as one unit. The two cells commonly have one bit line. Thetwo cells are required to have identical characteristics. However, dueto the above misalignment, the two cells may show differentcharacteristics. Therefore, the buried insulating layers and the gateelectrodes require an alignment margin.

As a result, with a high integration of a semiconductor device, usingthe method disclosed in the U.S. Pat. No. 6,429,091, there is alimitation in forming DRAM cells being capable of minimizing a leakagecurrent and having an alignment margin.

SUMMARY OF THE INVENTION

Therefore, the present invention is directed to provide a semiconductordevice such as a DRAM cell employing a patterned buried insulatinglayer, to minimize a leakage current and to ensure an alignment marginof the buried insulating layer and the gate electrode.

Further, the invention is directed to provide a method of fabricating asemiconductor device such as a DRAM cell employing an asymmetricalburied insulating layer with respect to a transistor, to minimize aleakage current.

In accordance with an exemplary embodiment, the present inventionprovides a DRAM cell employing an asymmetrical buried insulating layer.The DRAM cell includes a lower semiconductor substrate. An upper siliconpattern is located on the lower semiconductor substrate. The uppersilicon pattern includes a channel region, and a source region and adrain region spaced apart from each other by the channel region.Further, a gate electrode is electrically insulated from the uppersilicon pattern and intersects over the channel region. Further, a bitline and a cell capacitor are electrically connected to the sourceregion and the drain region, respectively. A buried insulating layer isinterposed between the drain region, electrically connected to the cellcapacitor, and the lower semiconductor substrate. The buried insulatinglayer has an extension portion partially interposed between the channelregion and the lower semiconductor substrate. Therefore, an alignmentmargin can be ensured in order to align the gate electrode on the buriedinsulating layer, and a leakage current of a cell capacitor can beprevented as well as a floating body effect.

Preferably, the upper silicon pattern may be a silicon epitaxial layer.Further, the buried insulating layer may be a silicon oxide layer, orstacked layers consisting of a silicon oxide layer and a silicon nitridelayer.

Further, the source region and the drain region are determined inaccordance with the operation of a DRAM cell, and hereinafter, animpurity region, electrically connected with the bit line, is defined asthe source region, and another impurity region, electrically connectedwith the cell capacitor, is defined as the drain region. The sourceregion preferably has an impurity concentration higher than that of thedrain region. Further, the source region has a junction depth greaterthan that of the drain region, and can be extended to the lowersemiconductor substrate. Therefore, junction resistances of the bit lineand the source region can be reduced, thereby improving the operationspeed.

In accordance with another exemplary embodiment, the present inventionprovides a method of fabricating a DRAM cell employing an asymmetricalburied oxide layer. The method includes forming a sacrificial layer on alower semiconductor substrate. Patterning the sacrificial layer, anopening exposing the lower semiconductor substrate is formed, and anupper silicon epitaxial layer is formed on the semiconductor substratehaving the opening.

Then, patterning the upper silicon epitaxial layer, the sacrificiallayer, and the lower semiconductor substrate, a trench is formed fordefining an active region. The active region includes an upper siliconpattern intersecting over the exposed lower semiconductor substrate, andsacrificial layer patterns exposed on sidewalls of the trench. Then,selectively removing the exposed sacrificial layer patterns, and forminga buried insulating layer and an isolation layer, respectively, to fillempty spaces, from which the sacrificial layer patterns are removed, andthe trench. Gate electrodes are formed on the semiconductor substratehaving the isolation layer formed thereon, insulated from the uppersilicon pattern, and intersecting the upper silicon pattern. The gateelectrodes are formed spaced apart from one another and passing overopening sidewalls that face each other. N-type impurity ions areimplanted using the gate electrodes as ion implantation masks to form asource region and a drain region. The drain region is located above theburied insulating layer. Then a bit line and a cell capacitor are formedthat are electrically connected with the source region and the drainregion, respectively.

Preferably, the sacrificial layer may be an SiGe epitaxial layer. Afterforming the SiGe epitaxial layer, a lower silicon epitaxial layer may beformed on the SiGe epitaxial layer.

Further, the opening for exposing the semiconductor substrate may be ahole or a line-shaped groove. In the case that the opening is a hole, anarrow width of the upper silicon pattern is preferably confined to anupper surface of the semiconductor substrate exposed via the hole.Therefore, the formation of the buried oxide layer under the sourceregion is prevented.

Further, the gate electrodes are formed to have a width being smallerthan that of the opening. That is, the opening is formed to have agreater width than that of the gate electrode. Thus, the opening can beformed relatively easily.

Preferably, before or after forming the source/drain regions, aphotoresist pattern having an opening is formed on the semiconductorsubstrate having the gate electrodes formed thereon, and the openingexposes an upper surface of the active region between the gateelectrodes. The active region between the gate electrodes means a sourceregion. Then, N-type impurity ions are injected using the photoresistpattern and the gate electrodes as ion implantation masks. Therefore,since an impurity ion concentration of the source region is higher thanthat of the drain region, a junction resistance of the bit line can bereduced.

According to embodiments of the present invention, a floating bodyeffect problem can be solved, and leakage current characteristics of thesemiconductor device can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the invention will becomemore apparent to those of ordinary skill in the art by describing indetail preferred embodiments thereof with reference to the attacheddrawings in which:

FIG. 1 is a sectional view to illustrate a DRAM cell according to oneembodiment of the invention.

FIG. 2 depicts a layout to illustrate a method of fabricating a DRAMcell according to a preferred embodiment of the invention.

FIGS. 3 to 11 are sectional views taken along a line of I-I of FIG. 2 toillustrate a method of fabricating a DRAM cell according to a preferredembodiment of the invention.

FIG. 12 is a graph showing the leakage currents of a specimen fabricatedaccording to a preferred embodiment of the invention, to illustrate theleakage current characteristics of a DRAM cell, in which theinterposition locations of respective buried insulating layers in thespecimen are varied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout thespecification.

FIG. 1 is a sectional view to illustrate a DRAM cell according to oneembodiment of the invention.

Referring to FIG. 1, an upper silicon pattern 32 a is located on a lowersemiconductor substrate 21. The lower semiconductor substrate 21 may bea single crystal silicon substrate. The upper silicon pattern 32 a maybe a silicon epitaxial layer. An isolation layer 39 may cover thesidewalls of the upper silicon pattern 32 a. The isolation layer 39defines an active region of a DRAM cell. The isolation layer 39 isextended into the lower semiconductor substrate 21.

The upper silicon pattern 32 a includes a channel region 47 c, and asource region 47 s and a drain region 47 d spaced by the channel region47 c. In the DRAM cell using an NMOS transistor, the source/drainregions 47 s, 47 d are N-type impurities-doped regions and the channelregion 47 c is a P-type impurities-doped region.

Preferably, a concentration of impurities of the source region 47 s maybe higher than that of the drain region 47 d. Further, the source region47 s may be vertically extended to include a portion of the lowersemiconductor substrate 21.

Further, a gate electrode 43 is electrically insulated from the uppersilicon pattern 32 a, and is located on top of the channel region 47 c.The gate electrode 43 is insulated from the upper silicon pattern 32 aby a gate insulating layer 41. Further, spacers 49 may cover thesidewalls of the gate electrode 43.

A bit line BL is electrically connected to the source region 47 s, and acell capacitor CC is electrically connected to the drain region 47 d.The bit line BL functions to convey charges and the cell capacitor CCfunctions to store charges. Further, the source region 47 s may have animpurity concentration being higher than that of the drain region 47 d.That is, a junction resistance of the bit line BL may be lower than thatof the cell capacitor CC, and an operation speed of a DRAM cell may beimproved.

A buried insulating layer 37 is interposed between the drain region 47 dand the lower semiconductor substrate. The buried insulating layer 37may be a silicon oxide layer (SiO₂), and may be stacked layers of thesilicon oxide layer and a silicon nitride layer (SiN). Further, theburied insulating layer 37 may include an empty space. The buriedinsulating layer 37 may have an extension portion which is partiallyinterposed between the channel region 47 c and the lower semiconductorsubstrate 21. That is, the extension portion is shorter than a length ofthe channel region 47 c. The length of the extension portion ispreferably equal to or shorter than ½ a length of the channel region 47c in the longitudinal direction of the channel region. Thus, the channelregion 47 c may be physically connected with the lower semiconductorsubstrate 21. Thus, the DRAM cell can prevent the generation of afloating body effect suffered by the transistor having an SOI structure.

Further, the buried insulating layer 37 reduces a junction area of thedrain region 47 d. Further, the extension portion of the buriedinsulating layer 37 reduces an intensity of the electric field generatedbetween the drain region 47 d and the channel region 47 c. The reductionof the junction area of the drain region 47 d, and the reduction of theintensity of the electrical field lead to the reduction of a leakagecurrent. Thus, while the cell capacitor CC stores charges, the leakagecurrent may be prevented.

Further, the effect on leakage currents due to the length of theextension portion is smaller than the effect on leakage currents due tothe junction area of the drain region 47 d. Therefore, when theextension portion is formed with an appropriate length, the alignmentmargin of the buried insulating layer 37 and the gate electrode 43 maybe increased.

Now hereinafter, a method of fabricating a DRAM cell according to apreferred embodiment of the invention will be described in reference toaccompanying drawings.

FIG. 2 depicts a layout to illustrate a method of fabricating a DRAMcell according to a preferred embodiment of the invention, and FIGS. 3to 11 are sectional views taken along a line of I-I of FIG. 2 toillustrate a method of fabricating a DRAM cell according to a preferredembodiment of the invention.

Referring to FIGS. 2 and 3, a sacrificial layer 23 is formed on a lowersemiconductor substrate 21. The lower semiconductor substrate 21 may bea single crystal silicon substrate. Further, the sacrificial layer 23 isformed of a material having a lattice constant equal to or closer tothat of silicon. Preferably, the sacrificial layer 23 may be an SiGeepitaxial layer. A lower silicon epitaxial layer 25 may be sequentiallyformed on the sacrificial layer 23. The lower silicon epitaxial layer 25prevents the upper surface of the sacrificial layer 23 from beingexposed in a subsequent process.

Referring to FIGS. 2 and 4, the lower silicon epitaxial layer 25 and thesacrificial layer 23 are patterned, so as to form an opening 27 exposingthe lower semiconductor substrate 21. A hard mask layer (not shown) maybe formed on the lower silicon epitaxial layer 25 to form the opening27. The hard mask layer is patterned, so as to form a hard mask patternhaving an opening exposing the lower silicon epitaxial layer 25. Then,using the hard mask pattern as an etch mask, the lower silicon epitaxiallayer 25 and the sacrificial layer 23 are sequentially etched, so as toform an opening 27 exposing the lower semiconductor substrate 21. Then,the hard mask pattern is removed.

As shown in FIG. 2 with a dotted line, the opening 27 may be aline-shaped groove 27 g or hole 27 h. The opening 27 may be formed tohave a predetermined width Wo in one direction. The sacrificial layer 23and the lower silicon epitaxial layer 25 may be exposed on the sidewallsof the opening 27.

Referring to FIGS. 2 and 5, an upper silicon epitaxial layer 31 isformed on the semiconductor substrate having the opening 27 formedthereon. The upper silicon epitaxial layer 31 fills the opening 27 onthe exposed top of the semiconductor substrate 21, and covers the uppersurface of the lower silicon epitaxial layer 25. If the sacrificiallayer 23 is formed of an SiGe epitaxial layer, the upper siliconepitaxial layer 31 may be uniformly formed on the SiGe epitaxial layer.Therefore, a defect-free silicon epitaxial layer may be formed insidethe opening 27. Thus, the upper surface of the upper silicon epitaxiallayer 31 may be formed flat, and may be formed to have a step differencedue to the opening 27.

Referring to FIGS. 2 and 6, the upper silicon epitaxial layer 31, thelower silicon epitaxial layer 25, the sacrificial layer 23, and thelower semiconductor substrate 21 are sequentially patterned, so as toform a trench 35 to define the active region. The active region includesan upper silicon pattern 32 a intersecting over the lower semiconductorsubstrate 21 exposed by the opening 27, and the sacrificial layerpattern 23 a exposed on the sidewalls of the trench 35. The uppersurface of the upper silicon pattern 32 a will be the upper surface ofthe active region.

The upper silicon pattern 32 a includes lower silicon epitaxial patterns25 a and upper silicon epitaxial patterns 31 a. Further, the sacrificiallayer patterns 23 a are formed under the lower silicon epitaxial layerpatterns 25 a. The sacrificial layer patterns 23 a may be formed to faceeach other about the opening 27.

Preferably, a hard mask pattern 33 may be formed in order to form thetrench 35. Using the hard mask pattern 33 as an etch mask, the uppersilicon epitaxial layer 31, the lower silicon epitaxial layer 25, thesacrificial layer 23 and the lower semiconductor substrate 21 areetched, so as to form the trench 35.

Referring to FIGS. 2 and 7, the exposed sacrificial layer patterns 23 aare selectively etched and removed. The sacrificial layer patterns 23 aare selectively etched using a wet etch technology. Since thesacrificial layer patterns 23 a are removed, empty spaces 23 b areformed where the sacrificial layer patterns 23 a are previously formed.

Referring to FIGS. 2 and 8, a buried insulating layer 37 is formed inthe empty spaces 23 b from which the sacrificial layer patterns 23 a areremoved. The buried insulating layer 37 may be formed of empty spaces,the silicon oxide layer, or the silicon nitride layer, and may be formedby stacking these layers. Preferably, a silicon oxide layer is formed tocover the inner sidewalls of the empty spaces 23 b, and then, a siliconnitride layer is formed. As a result, the silicon nitride layer isenclosed by the silicon oxide layer, so as to form a buried insulatinglayer 37. Herein, the insulating layers may be formed on the inner wallsof the trench 35. Then, an insulating layer is formed to fill the trench35, and the hard mask pattern 33 is planarized until its upper surfaceis exposed, so as to form an isolation layer 39 to fill the trench 35.The isolation layer 39 covers the sidewalls of the upper silicon pattern32 a and the sidewalls of the buried insulating layer 37. Herein, thehard mask pattern 33 functions to protect the upper surface of the uppersilicon pattern 32 a. Further, after the empty spaces 23 b are formed,the isolation layer 39 can be formed without filling the empty spaces 23b. Preferably, before the isolation layer 39 is formed, a hightemperature oxide layer can be formed on the sidewalls of the emptyspaces 23 b and the sidewalls of the trench 35. The high temperatureoxide layer protects the sidewalls of the empty spaces and the sidewallsof the trench 35. As a result, the buried insulating layer 37 includesthe empty space.

Referring to FIGS. 2 and 9, after the isolation layer 39 is formed, thehard mask pattern 33 is removed. Then, gate electrodes 43 are formedinsulated from the upper silicon pattern 32 a and intersecting the uppersilicon pattern 32 a. The gate electrodes 43 may be insulated from theupper silicon pattern 32 a by the gate insulating layer 41. Further, thegate electrodes 43 are formed to pass over opening sidewalls facing eachother. Preferably, the gate electrodes 43 may be formed to have a widthsmaller than that of the opening 27. That is, the opening 27 may beformed to have a width being relatively greater than those of the gateelectrodes 43. Thus, the opening 27 can be easily formed, by patterningthe sacrificial layer 23.

Further, before the gate electrodes 43 are formed, P-type channel ionscan be injected. The channel ions may be implanted in order to control athreshold voltage.

Referring to FIGS. 2 and 10, a photoresist pattern 45 may be formed onthe semiconductor substrate having the gate electrodes 43 formedthereon, so as to expose the active region between the gate electrodes43. N-type impurity ions are implanted using the photoresist pattern 45and the gate electrodes 43 as ion implantation masks, so as to formanother impurity region 47 a inside the active region.

Referring to FIGS. 2 and 11, N-type impurity ions are implanted insidethe active region using the gate electrodes 43 as ion implantationmasks, so as to form the source region 47 s and the drain region 47 d.The source/drain regions 47 s, 47 d may be formed using a typical LDDprocess. That is, low concentration impurity regions are formed byimplanting N-type impurity ions using the gate electrodes 43 as ionimplantation masks. Then, the spacers 49 are formed to cover thesidewalls of the gate electrodes 43, and using the spacers 49 and thegate electrodes 43 as ion implantation masks, N-type impurity ions areimplanted, so as to form high concentration impurity regions.

The process of forming another impurity region 47 a (FIG. 10) may beperformed after the operation of forming the source/drain regions 47 s,47 d. Further, when another impurity region 47 a is formed, the sourceregion 47 s has a relatively high impurity ion concentration and agreater junction depth compared with the drain regions 47 d.

An interlayer insulating layer (not shown) is formed on thesemiconductor substrate having the source/drain regions formed thereon,and contact plugs (not shown) are formed penetrating the interlayerinsulating layer and connected with the source/drain regions 47 s, 47 d.Then, a bit line BL is formed to be connected with the source region 47s via the contact plug. Further, a cell capacitor CC is formed to beelectrically connected with the drain regions 47 d via the contactplugs. The bit line BL and the cell capacitors CC are electricallyinsulated from each other.

According to one preferred embodiment of the invention, a buriedinsulating layer 37 may be formed under the drain region 47 delectrically connected with the cell capacitor CC. The buried insulatinglayer 37 extends under the gate electrodes 43. Therefore, the chargeleakage of the cell capacitor CC can be prevented, and the alignmentmargin of the buried insulating layer 37 and the gate electrodes 43 canbe ensured. Further, the opening 27 may be formed to have a widthgreater than those of the gate electrodes 43. Thus, the opening 27 canbe easily formed, by patterning the sacrificial layer 23.

FIG. 12 is a graph showing the leakage currents of a specimen toillustrate the leakage current characteristics of a DRAM cell, in whichthe interposition locations of respective buried insulating layers inthe specimen are varied. Herein, the structural differences of thespecimen are presented in Table 1. TABLE 1 Specimen A Specimen BSpecimen C Specimen D 1. formation of a X O O O buried insulating layer2. location of a buried — Under Under Under drain insulating layersource/drain source/drain and region and regions channel regions channelregion 3. overlap extent of a — — 0.5 Wg 0.5 Wg buried insulating layerand a gate electrode 4. minimum distance — Wg 0.5 Wg 2 Wg between buriedinsulating layers

Referring to Table 1, specimen A is a planar transistor without a buriedinsulating layer, and specimen B is fabricated, in which buriedinsulating layers are spaced as much as the width of the gate electrode,and formed under the source/drain regions. Specimen C is fabricated, inwhich buried insulating layers are spaced as much as ½ the width Wg ofthe gate electrode, and formed under the source/drain regions and thechannel region. Further, specimen D is fabricated, in which a buriedinsulating layer is located under the drain region, and is partiallyextended to the channel region. Each specimen employs a drain electrodeinstead of a cell capacitor CC.

In the specimen B, C, and D employing the buried insulating layer, theminimum distance between the buried insulating layers is the greatest inthe specimen D. Further, the minimum distance between the buriedinsulating layers is related with a width Wo of the opening 27 (FIG. 4).Thus, among the specimen B, C, and D, the formation of the buriedinsulating layer is easiest in the specimen D.

Further, in the specimen, the source region and the lower semiconductorsubstrate are grounded, and a drain voltage V_(DS) is fixed. Whileincreasing a gate voltage V_(GS), an electric current IDS flowing in thedrain electrode is measured.

Referring to FIG. 12, when the drain voltage V_(DS) is 2.5 V, anoff-state current of the specimen A is greatest, and an off-statecurrent of the specimen C is least. Further, the specimen B and D do notshow a great difference in off-state current, but the specimen D shows aslightly low off-state current. The specimen A shows a relatively highoff-state current at 0.05 V of the drain voltage.

As a result, employing buried insulating layers, the off-state currentcan be reduced, and since the buried insulating layers are extended tothe channel region, the off-state current can be further reduced.

The off-state current is related with a leakage of the charges stored inthe capacitor of the DRAM cell. Thus, a buried insulating layer isformed under the drain region into which the capacitor is electricallyconnected, thereby preventing a leakage current of the capacitor.

According to the invention, a patterned buried insulating layer isemployed near a drain region into which a cell capacitor is electricallyconnected, thereby providing a DRAM cell being capable of ensuring analignment margin of the buried insulating layer and the gate electrode,and minimizing a leakage current. Further, the buried insulating layeris formed using a sacrificial layer such as an SiGe epitaxial layer,thereby providing a method of fabricating a DRAM cell being capable ofensuring an alignment margin of the buried insulating layer and the gateelectrodes, and minimizing a leakage current.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention as defined in the following claims.

1. A semiconductor device comprising: a lower semiconductor substrate;an upper silicon pattern located on the lower semiconductor substrate,and the upper silicon pattern including a channel region, a sourceregion and a drain region spaced apart from each other by the channelregion; a gate electrode electrically insulated from the upper siliconpattern and extending across the channel region; a bit line and a cellcapacitor electrically connected to the source region and the drainregion respectively; and a buried insulating layer interposed betweenthe drain region and the lower semiconductor substrate, the buriedinsulating layer having an extension portion partially interposedbetween the channel region and the lower semiconductor substrate.
 2. Thesemiconductor device according to claim 1, wherein the upper siliconpattern comprises a silicon epitaxial layer.
 3. The semiconductor deviceaccording to claim 1, wherein the source region has an impurityconcentration higher than that of the drain region.
 4. The semiconductordevice according to claim 1, wherein the source region is extended to aninner portion of the lower semiconductor substrate.
 5. The semiconductordevice according to claim 1, wherein the extension portion has a lengthequal to or smaller than about half the length of the channel region. 6.The semiconductor device according to claim 1, wherein the buriedinsulating layer comprises an empty space.
 7. The semiconductor deviceaccording to claim 1, wherein the buried insulating layer comprisessilicon oxide.
 8. The semiconductor device according to claim 1, whereinthe buried insulating layer comprises a silicon oxide layer and asilicon nitride layer, which are stacked.
 9. The semiconductor deviceaccording to claim 1, wherein the semiconductor device comprises a DRAMcell.
 10. A method of fabricating a semiconductor device, the methodcomprising: a) forming a sacrificial layer on a lower semiconductorsubstrate; b) patterning the sacrificial layer to form an opening toexpose a portion of the lower semiconductor substrate; c) forming anupper silicon epitaxial layer on the lower semiconductor substratehaving the opening; d) patterning the upper silicon epitaxial layer, thesacrificial layer, and the lower semiconductor substrate to form atrench that defines an active region, the active region including anupper silicon pattern extending across the exposed lower semiconductorsubstrate, and sacrificial layer patterns exposed on sidewalls of thetrench; e) selectively removing the exposed sacrificial layer patterns;f) forming a buried insulating layer and an isolation layer,respectively, filling a space, from which the sacrificial layer patternsare removed, and the trench; g) forming gate electrodes on the lowersemiconductor substrate having the isolation layer formed thereon,insulated from the upper silicon pattern, and intersecting the uppersilicon pattern, the gate electrodes spaced apart from one another andpassing over opening sidewalls that face each other; h) forming a sourceregion and a drain region along opposite sides of the gate electrodes,the drain regions located over the buried insulating layers; and i)forming a bit line and a cell capacitor being electrically connectedwith the source region and the drain region, respectively.
 12. Themethod according to claim 10, wherein the gate electrodes are formed tohave a width smaller than that of the opening.
 13. The method accordingto claim 10, further comprising, before or after forming the sourceregion and the drain region: forming a photoresist pattern having a holeon the lower semiconductor substrate having the gate electrodes formedthereon, the hole exposing an upper surface of the active region betweenthe gate electrodes; and implanting N-type impurity ions using thephotoresist pattern and the gate electrodes as ion implantation masks.14. The method according to claim 10, wherein the sacrificial layercomprises an SiGe epitaxial layer.
 15. The method according to claim 14,further comprising forming a lower silicon epitaxial layer on the SiGeepitaxial layer.
 16. The method according to claim 10, wherein theopening for exposing a portion of the semiconductor substrate is agroove.
 17. The method according to claim 10, wherein the opening forexposing a portion of the semiconductor substrate is a hole.
 18. Themethod according to claim 17, wherein a narrow width of the uppersilicon pattern is confined to an upper surface of the semiconductorsubstrate exposed through the hole.
 19. The method according to claim10, wherein the buried insulating layer comprises an empty space. 20.The method according to claim 10, wherein the semiconductor devicecomprises a DRAM cell.
 21. The method according to claim 10, wherein theburied insulating layer comprises silicon oxide.
 22. The semiconductordevice according to claim 10, wherein the buried insulating layercomprises a silicon oxide layer and a silicon nitride layer, which arestacked.